HIV-1 Vpu enhances the release of virions from infected cells by overcoming a cellular inhibitor that retains nascent virions within and on infected cells. The identity of this inhibitor has recently been revealed: it is the transmembrane, GPI-anchored protein BST-2, also known as HM1.24, CD317, or tetherin. BST-2 seems able to affect diverse enveloped virions, suggesting a broad role in the host defense against viruses including HIV-1. The research proposed here has three specific aims: 1) to reveal how BST-2 retains HIV-1 virions on infected cells; 2) to determine how Vpu antagonizes this restriction; 3) to understand the regulation of BST-2 during the innate immune response and to explore the potential function of BST-2 in antigen presenting cells. These aims will be pursued using a concerted experimental approach including targeted mutagenesis of BST-2 and Vpu, characterization of the interaction between BST-2 and Vpu, analysis of the effects of Vpu on the intracellular trafficking and virion-incorporation of BST-2, and analysis of the regulation and function of BST-2 in primary T lymphocytes and antigen presenting cells. When these aims are completed, we will know how BST-2 retains virions on infected cells, how Vpu counteracts this protein, how BST-2 is regulated during the innate immune response, and whether it plays a role in the uptake of virions by antigen presenting cells during the adaptive immune response.